Existing computed tomography (CT) scanners rely on a radiation source and a detector that rotate around the patient and observe the attenuation of the beam as it passes through the patient from a variety of directions. From this data, a three dimensional representation of the internal structure of the patient is computed.
These fall into two distinct groups. First, the simple CT scanner directs a narrow “pencil” beam through the patient. Each complete rotation results in a two dimensional image in the form of a “slice” or section through the patient. The scanner (or the patient) is then indexed along the axis of rotation and a further slice is scanned. These slices can then be assembled into a three dimensional image.
Second, a cone beam CT scanner directs a divergent beam toward the patient to produce (at any selected time) a two dimensional projection of the entire region of interest. As the beam rotates, projections are acquired from different directions and a three-dimensional image can be constructed.
Cone beam CT is in general advantageous as compared to simple CT since its resolution is the same in all directions. Simple CT has a high resolution in the plane of each slice, but the resolution perpendicular to this is limited by the index distance.
Both techniques assume that the patient is static. This is an invalid assumption for a living patient, as some parts such as the heart, lungs and diaphragm will inevitably be moving. As the patient breathes, structures around the lungs and diaphragm will move and this presents difficulties in obtaining good quality scans. In simple CT, artefacts in the image arise; as the slice is indexed along the patient, the breathing cycle can move structures into and out of the slice being scanned at that time. Periodic artefacts thus develop in the reconstructed volume. In cone beam CT, some artefacts result from the reconstruction process, but the main problem is a blurring of the image in the form of an averaging process.
In the treatment of lung tumours (for example) it is important to know the position of the tumour and its movement as the breathing cycle progresses. The time-averaged information derived from cone beam CT is inadequate, as this cannot distinguish between a small tumour that dwells in one area and a large tumour that briefly passes an area.
Hitherto, when performing CT scans of the thorax, patients have been asked to control their breathing in accordance with an external stimulus, or a proxy for the phase of the breathing cycle has been detected. Examples of proxies that have been used include the local temperature around the nostril, and the dimensions of the thorax. These have proved to be of assistance but generally unsatisfactory.